In an internal combustion engine, lubricating oil is heated and exposed to blowby gas during long-time operation of the engine, and unburned products of hydrocarbons and denatured products of oil additives are contained in the lubricating oil. In a diesel engine, carbon particulates are also contained in lubricating oil. These unburned products, denatured products of oil additives, and carbon particulates are generally collectively referred to as “oil sludge.” If such oil sludge adheres to components of an engine and is then deposited thereon, these components may be worn, and passages of the lubricating oil may be clogged, so that the functions of the engine components such as oil rings may be hindered.
Referring now to FIGS. 1 and 2, the above problem in the structures of two typical oil rings will be described in more detail.
FIG. 1 shows a cross-section of a part (the right half) of a coil spring loaded oil control ring 1 (a two-piece oil ring). The two-piece oil ring 1 includes: an annular oil ring body 4 having an end gap and composed of a pair of axially disposed upper and lower rail portions 2 and a web 3 connecting the rail portions 2; and a coil expander 5 that presses the oil ring body 4 outwardly in its radial direction. An inner peripheral groove 6 is formed on the inner peripheral side of the oil ring body 4, and an outer peripheral groove 8 is formed on the outer peripheral side. Oil drainage passages 7 spaced apart at predetermined circumferential intervals are formed in the web 3 and pass radially therethrough.
In such a two-piece oil ring 1, oil sludge is deposited on the outer peripheral surface of the coil expander 5, between the pitches thereof, and on the inner peripheral groove 6 of the oil ring body 4. This may cause the passages of the lubricating oil to be clogged. The oil sludge is also deposited on the oil drainage passages 7 and the outer peripheral groove 8, and the oil drainage passages 7 may thereby be clogged. If the passages of the lubricating oil such as the oil drainage passages 7 are clogged, the function of controlling the oil is hindered, and the consumption of the lubricating oil may increase. If the oil sludge is deposited between the pitches of the coil expander 5, the elasticity of the coil expander 5 may be lost. In particular, when a low-tension coil expander 5 is used, the force of pressing the oil ring body 4 against the inner wall surface of a cylinder is reduced due to the oil sludge deposited between the pitches of the coil expander 5, and the conformability of the two-piece oil ring 1 to the cylinder wall surface may be reduced.
FIG. 2 shows a cross-section of a part (the right half) of an expander/segment oil control ring 10 (a three-piece oil ring). This three-piece oil ring 10 includes a pair of annular side rails 11 having end gaps and a spacer expander 12 that supports the side rails 11. Tabs 13 are provided on the inner peripheral side of the spacer expander 12, and outer protruding portions 16 that support the side rails 11 are provided on the outer peripheral side. Flat base dents 14 are provided in sections connecting the tabs 13 and the outer protruding portions 16.
When the spacer expander 12 is combined with the side rails 11, spaces 15 surrounded by the tabs 13, the outer protruding portions 16, the base dents 14, and the side rails 11 are formed. In the three-piece oil ring 10, the side rails 11 are pressed by radial and axial component forces caused by the angles of the tabs 13 of the spacer expander 12, thereby exerting a sealing function on the wall surface of a cylinder and on the upper and lower surfaces of a ring groove. In particular, a narrow-width three-piece oil ring having a narrow axial width, i.e., a reduced dimension h1, has good conformability to the cylinder wall surface and also has a side sealing function. Therefore, even when the tension of this oil ring is small, friction loss can be reduced without an increase in oil consumption. However, even in such a three-piece oil ring 10, oil sludge is more likely to be deposited in the spaces 15 surrounded by the side rails 11 and the base dents 14 on the outer peripheral side of the tabs 13 of the spacer expander 12. In particular, when the axial width of the three-piece oil ring is made small, the deposition of oil sludge may cause sticking of the side rails 11 to the spacer expander 12. In this case, the conformability of the side rails 11 to the inner peripheral surface of the cylinder is lowered, and the oil consumption is likely to increase.
To prevent the above-described adhesion and deposition of oil sludge onto an oil ring, in some conventional methods, liquid repellent treatment (for example, a fluorine-containing coating) is used. More specifically, an oil repellent coating is formed on the surface of the oil ring to prevent adhesion of oil sludge in lubricating oil. Examples of the material for the fluorine-containing coating used for the oil repellent treatment include polytetrafluoroethylene and fluoro alkylsilane. For example, Patent Document 1 proposes a method of forming a liquid repellent film by a sol-gel method using a metal alkoxide and a fluoroalkyl group-substituted metal alkoxide produced by substituting a fluoroalkyl group for a part of the alkoxy group (alkoxyl group) in the metal alkoxide. It is known that a material containing a fluoroalkyl group has water repellency and oil repellency. Therefore, by providing a coating film having fluoroalkyl groups on its surface, liquid repellency is imparted to engine components to prevent adhesion and deposition of oil sludge. Patent Document 2 discloses a technique for improving the effect of preventing adhesion and deposition of oil sludge by using a fluorine-containing coating having an increased thickness. To increase the thickness of the coating, the polymerization of a fluoroalkyl group-substituted alkoxide is promoted before a coating solution is applied to a substrate.
Patent Document 3 describes that, when components of an internal combustion engine are coated with a carbon-based film having a predetermined surface free energy and a predetermined coating roughness, repellency to deposits (oil sludge) is improved. Therefore, deposition of the deposits and sticking are suppressed, and efficient combustion operation is maintained with small performance degradation. Polypropylene resins, perfluoroethylene propylene (FEP) resins, polytetrafluoroethylene (PTFE), fluoro alkylsilanes, and the like are exemplified as the carbon-based film.
As described above, to prevent adhesion and deposition of oil sludge, the use of oil repellent treatment of the surface of an oil ring using, for example, a fluorine-containing thin film has been contemplated. However, since an engine in operation is exposed to high temperature, it has been found that the fluorine-containing thin film is thermally decomposed during long-time operation, so that the effect of preventing adhesion and deposition of oil sludge is reduced. The fluorine-containing thin film may eventually disappear, and the effect of preventing adhesion and deposition may thereby be lost. Therefore, at present, an oil ring for an internal combustion engine has not been obtained which has a high heat resistant coating capable of maintaining the effect of preventing adhesion and deposition of oil sludge even during long-time operation.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-27995    [Patent Document 2] Japanese Patent Application Laid-Open No. Hei. 10-157013    [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-291884